Vaillant F.2 fault codewhat it means and how to fix it
The hot water outlet sensor circuit is open. On a combi that is the sensor on the water leaving for your taps; on an ecoTEC plus storage combi it is the shift-load cylinder sensor.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.2
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What F.2 means
F.2 is the open-circuit partner of F.12, and which sensor it refers to depends on the boiler. The 2005 ecoTEC servicing instructions print it as "domestic hot water outlet NTC open circuit (combination boiler only) — NTC connector not plugged in or loose, NTC defective, connector on the electronics not plugged in correctly, cable defective", which is the plain combi's outlet sensor.
The ecoTEC plus ..6/5-5 table reads "interruption: DHW outlet sensor — shift-load cylinder temperature sensor defective or not connected", to be checked "on the shift-load cylinder, only in conjunction with F.91", which ties it to the VUI storage combi and the actoSTOR module rather than a plain combi. The exclusive table carries the same row. The 2022 CS/1-5 returns to the simpler reading as F.002, "the signal for the domestic hot water connection temperature sensor has been interrupted", with the wiring harness and the sensor as its two causes.
In every version the board is reading nothing where it expects a temperature, so it cannot regulate hot water and stops producing it. The check list is the plug, the cable harness and the sensor, all inside the case, and the pro, ecoFIT, ecoTEC sustain and legacy tables do not carry the row at all.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor plug not connected or loose | Vaillant's first cause in 2005 and still the first check. A connector disturbed during work on the plate heat exchanger or the pipework beside it. | Common |
| Outlet or cylinder sensor failed open | The thermistor's element or lead has broken. A clip-on or immersion NTC, inexpensive as a part. | Common |
| Cable harness break | A damaged lead between sensor and board. On the storage combi the run to the shift-load cylinder sensor is longer and more exposed than a combi's. | Occasional |
| Board connector not seated | The 2005 wording's "connector on the electronics not plugged in correctly". Checked before any part is ordered. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£170
Time on site
45 minutes
A reseated plug is a call-out of £80 to £120; a replacement sensor is £110 to £170 fitted.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Vaillant flow, return or cylinder temperature sensor (NTC)
£12–£38Authorised Vaillant parts distributor
Price checked 28 September 2026
Vaillant wiring harness (plug-and-cable set to a sensor or actuator)
£20–£65Authorised Vaillant parts distributor
Price checked 28 September 2026
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Related Vaillant fault codes
- F.0Get it checkedThe flow temperature sensor circuit is broken. The boiler cannot read how hot the water leaving it is, so it has stopped rather than fire blind.Related: temperature sensors
- F.72Get it checkedThe flow and return sensors are reporting a temperature difference that cannot be real. The boiler is blaming the sensors themselves rather than the water.Related: temperature sensors
- F.1Get it checkedThe return temperature sensor circuit is open: the boiler is getting no reading from the sensor on the pipe bringing water back in, so it has stopped.Related: temperature sensors
- F.10Get it checkedThe flow temperature sensor has short-circuited. The boiler is reading an impossible value from the sensor on its outlet pipe and will not fire on it.Related: temperature sensors
- F.11Get it checkedThe return temperature sensor has short-circuited. Vaillant treat it as a wiring or sensor fault, and the boiler stays off until the circuit reads correctly again.Related: temperature sensors
- F.84Get it checkedThe flow and return sensors disagree in a way that cannot be true — the return reading warmer than the flow. Vaillant's first explanation is that the two sensors have been swapped.Related: temperature sensors
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.